DESCRIPTION: Studies completed in this laboratory during the past several years have contributed to our understanding of two closely related questions in auditory research: 1) How do inhibitory amino acids function within known brainstem auditory circuits to encode acoustic information? 2) What is the impact of aging on inhibitory amino acid neurotransmission in the auditory brainstem? Studies of this nature have made significant inroads into the nature of presbycusis, a major disorder among the aged and may lead to pharmacological treatment strategies for age-related hearing loss. In vivo iontophoretic and in vitro studies in cochlear nucleus (CN) and inferior colliculus (IC) have revealed that inhibitory amino acid neurotransmitters are critically involved in the control of dynamic range, echo suppression and detection of signal in noise. Recent studies from this laboratory suggest that circuits within the CN and IC undergo age-related changes associated with inhibitory neurotransmission. The current proposal will extend this strong foundation of work by carrying out the following proposed studies: 1) microiontophoresis to examine the role of the inhibitory amino acid GABA in shaping responses to amplitude modulated sinusoids in the IC; 2) neurochemical techniques to further characterize age-related changes in the molecular nature of the GABAA receptor; and 3) single unit physiology to examine age-related changes in the response properties of DCN fusiform cells.